A field chopper or chopper forage harvester includes a comminution apparatus formed by a disc wheel chopper and at least one crushing device located downstream of the disc wheel chopper. The crushing device may be formed by at least one grinding and crushing roller cooperating with a fixed counterbase or at least two cooperating grinding and crushing rollers rotating in opposite directions.
A number of field choppers of this type are already known, note French Pat. No. 2 455 852 and No. E-OS 0 058 431, in which the comminution apparatus consists of a cylinder or drum chopper. The crushing device is located in a part of the cylinder housing positioned downwardly from a counterknife cooperating with the cylinder or drum. The material chopped by the cutting knives is moved downwardly along the circumferentially extending inner surface of the housing into the crushing device. Since separation of the chopped material is not effected over the short distance between the counterknife and the crushing device, the heavier particles do not move outwardly relative to the remainder of the material and the entire chopped material is accumulated at the opening into the crushing device and, as a result, it can be easily overloaded. The chopped material which is not accepted into the crushing device is accumulated in the path of the knives or blades on the cylinder and is conveyed directly into the discharge outlet without passing through the crushing device. As a result, a large part of the corn is not crushed and remains in its original chopped size. In the crushing device, on the other hand, the lighter and mostly fibrous chopped material components impede the crushing of the corn because of the resilience of such material. Relative to the chopping cylinder, since the crushing rollers have a much smaller diameter and, accordingly, a lower circumferential velocity, the lighter particles also interfere with the continued transport of the crushed material requiring additional transporting means, such as the blower used in the first patent mentioned above and the conveying worm employed in the second patent.